1. Field of the Invention
The present invention relates to RF communications and more particularly to diplexers and waveguide filters.
2. State of the Art
In a bidirectional RF communications system having a radio transceiver, rather than having separate transmit and receive antennas, a single transmit/receive antenna may be used. In such a system, a mechanism is required to separate transmit energy transmitted by the antenna from receive energy received by the antenna. A diplexer is a three-port RF device used for this purpose. A first port of the diplexer is coupled to a transmit signal source. A second port of the diplexer is coupled to the transmit/receive antenna, and a third port of the diplexer is coupled to a radio receiver.
Isolation between the receive and transmit ports is acheived by filters in each port. Another mechanism which can separate signal polarization is known as an orthogonal mode transducer (OMT). In a communications system the transmit signal may have a vertical or horizontal polarization, whereas the receive signal may have the opposite polarization. An OMT takes advantage of the different polarization characteristics of the transmit and receive signals such that the transmit signal may be injected into the antenna at the same time the receive signal is being extracted from the antenna without the transmit signal and the receive signal unduly interfering with each other.
Although an OMT by itself is able to achieve signal separation to a significant degree, signal separation may be improved using filters. If the transmit signal is set to a first frequency band and the receive signal is set to a second frequency band, a first bandpass filter may be used at the transmit port to ensure that transmissions are confined within the appropriate frequency band, and a second bandpass filter may be used at the receive port to reject energy not within the appropriate frequency band, including stray energy from the transmit port.
In the prior art, the foregoing arrangement has been realized using three separate assemblies, an OMT assembly, a first waveguide filter assembly, and a second waveguide filter assembly. One common type of waveguide filter assembly is a "post and screw" waveguide filter assembly. In a post and screw waveguide filter assembly, a waveguide body is provided having a channel and flanges at either end of the channel for connecting the waveguide filter assembly to other equipment. Screws are provided at locations determined during design of the filter so as to protrude into the channel an adjustable distance. The filter may be tuned by adjusting this distance for each of the screws. Once the filter has been tuned, the waveguide body and the screws may be encapsulated, thereby fixing the characteristics of the filter.
Using the foregoing components, an RF feed section of a radio transceiver is formed by connecting an OMT to two waveguide filter assemblies and connecting the waveguide filter assemblies in turn to a radio transceiver. This construction is bulky, expensive, and subject to mechanical and environmental damage or failure.